The present invention relates to improvements in tachometers and accelerometers, and particularly to such devices which sense velocity or rate of change of velocity from eddy current magnetic fields.
Devices for sensing the velocity of a non-magnetic, electrically-conductive, moving member by inducing eddy currents therein and sensing the strength of the resultant eddy current magnetic field have long been known. Such devices are exemplified by Kuhrt et al. U.S. Pat. No. 3,359,492, Dodgen et al. U.S. Pat. No. 4,441,077, and German published patent application No. 1,209,780, all of which utilize Hall effect sensors to sense the flux density of the eddy current field whose magnitude is directly proportional to the velocity of the moving member. The German reference also discloses sensing the rate of change of the flux density of the eddy current field using a coil, and thereby employing the device as an accelerometer.
In contrast, eddy current-type velocity sensors utilizing coils as field sensors and sensing velocity by detecting the frequency of passing eddy current fields, rather than the magnitude of their flux densities, have also been known as exemplified by Gallant U.S. Pat. No. 3,932,813 and Rickman, Jr. U.S. Pat. No. 4,439,728.
A problem with all of the foregoing devices is that they lack adequate magnetic circuit structures for their magnetic field sensors, and therefore are forced to sense a weak eddy current field. Because the eddy current field is not inherently strong, the magnitude of the eddy current flux density at the sensors is too low for sensing with an acceptable degree of reliability in the presence of interference from random noise and other stray potentials. The resultant low degree of sensitivity of the devices is particularly problematical in devices such as those of the Kuhrt et al., Dodgen et al., and German references which rely on detection of the relative magnitude of the eddy current flux density to produce a velocity-representative output signal.
The structural deficiency of all of the prior devices, which is primarily responsible for the probleof low sensitivity, is that the Hall or coil-type flux sensor is not incorporated into a high permeability magnetic circuit for collecting and concentrating eddy current flux to be detected by the sensor. In Kuhrt et al. the only magnetic circuit which includes the Hall sensor also includes the source magnets for the main magnetic field which, unless of a highly permeable permanent magnet material such as Alnico, would simply constitute gaps in the circuit, thereby precluding the use of all modern magnets having high energy products and high resistance to demagnetization, but low permeability, such as the ferrite and rare earth cobalt magnets. The comparable magnetic circuits of the other prior devices all contain large air gaps through which the moving member passes, thereby precluding a circuit of high permeability.
Moreover, where the only magnetic circuit available for the magnetic field sensors includes permanent magnets, as in Kuhrt et al. and Dodgen et al., a large number of boundaries of low permeability exist between the various components of the magnetic circuit, thereby further reducing its overall permeability and its effectiveness to concentrate the flux sensed by the sensor. In addition, the sensed eddy current flux is superimposed on the flux of the main permanent magnet field in such devices, thus worsening the reliability factor because of expected variations in the permanent magnet field.